Field studies indicating reduced activity of ivermectin on small strongyles in horses on a farm in Central Kentucky

Abstract

Field studies (n=6) were completed on evaluation of activity of ivermectin (200 μg/kg) paste formulation against small strongyles in horses (foals, yearlings, and older animals) on a farm (Farm MC) in Central Kentucky in late 2006 and during 2007. A girth tape was used to estimate body weights which were then used to calculate the proper dose rate of ivermectin. The foals, yearlings, and some of the older horses were born and raised on the farm. However, most of the older horses which were not raised on the farm had been there for several years. The horse herd was given ivermectin exclusively, usually four times a year, since 1990. An exception was that during the foal's period of life fenbendazole, pyrantel pamoate, and oxibendazole were given occasionally besides ivermectin. Efficacy of drug activity was determined by pretreatment and posttreatment counts of strongyle eggs per gram of feces (EPGs). Culture of strongyle eggs in feces from some of the horses showed that only small strongyle larvae were present. The research included two studies (A and B) in foals (n=24) and four studies (C, D, E, and F) in yearlings (n=13) alone or with older horses (n=10). For each of the studies (B through F), there was a treated and a nontreated group. These groups were switched for each treatment, i.e., the treated group in one study was the nontreated group in the next study and vice versa. Eggs per gram of feces counts were determined at 1-or 2-week posttreatment intervals for 4 weeks for study A and 6 weeks for studies B through F. Also, for studies B, E, and F, counts of EPGs were done either two or three times during the third week posttreatment. The studies showed a similar posttreatment pattern of strongyle EPG counts beginning to return at about 4 weeks and increasing at 5 and 6 weeks posttreatment. Two horses in study E and one in study F had low EPG values toward the end of the third week posttreatment. The results of this ivermectin investigation showed that the strongyle EPG counts started returning about twice as quickly post-ivermectin-treatment of horses than when the drug was first marketed in the early 1980s.

Monitoring the avermectin and pyrantel resistance status of nematode parasites of horses in the Czech Republic

“…It is known that adult parasites are more susceptible to anthelmintic effects than the developmental stages. The efficacy of oxibendazole (benzimidazole group) is 94-100% against ascarid adults but only 44.5% in 28-day-old larvae (Austin et al 1991;Lyons et al 2008). Thus, anthelmintic therapy 60-75 days post infection would be ideal to prevent pasture contamination and to reduce the ascarid population size.…”

“…One year later it was identified in Canada (Hearn and Peregrine 2003), followed by Texas (Craig et al 2007), Germany (von Samson-Himmelstjerna et al 2007), Denmark (Schoogard and Nielsen 2007) and Italy (Veronesi et al 2010). Ascarid resistance to pyrantel pamoate was noted in North America (Lyons et al 2008). Ascarid resistance to benzimidazoles has not yet been described (Brady and Nichols 2009).…”

Current state of anthelmintic resistance in horses with a special focus on the situation in Germany

“…Eine Feldstudie zeigte, dass bei einem Viertel der Pferdebestände in Deutschland (Resistenzhäufigkeit 25 %), Italien (Resistenzhäufigkeit 25 %) und Großbritannien (Resistenzhäufigkeit 25 %) eine Pyrantelresistenz der kleinen Strongyliden vorliegt und beim Fenbendazol (Benzimidazol) sogar die überwiegen-de Mehrheit der kleinen Strongyliden eine Resistenz aufweist (Resistenzhäufigkeit Deutschland 80 %) (Traversa et al 2009). Bezüglich der makrozyklischen Laktone liegen bisher keine Nachweise von Resistenzen vor, wobei jedoch im Zusammenhang mit Ivermectin und Moxidectin über eine verkürzte "EggReappearance-Period" (ERP) bei den kleinen Strongyliden berichtet wurde (Samson-Himmelstjerna et al 2007, Wirtherle et al 2004, Lyons et al 2008a, b, 2009, 2010und 2011. Vermutet wird, dass eine Verkürzung der ERP das erste Anzeichen einer sich entwickelnden anthelminthischen Resistenz ist (Sangster 1999).…”

Management factors and their impact on helminthic fecal egg count in horses

“…Ertelt et al Das Management der Pferde zeigte wie auch in anderen Studien (Larsen et al 2002, Hinney et al 2011b McBeath et al 1978, Borgsteede et al 1993, Parry et al 1993, Boersema et al 1995und 1998, DiPietro et al 1997, Demeulenaereet al 1997 Lyons et al 2008aLyons et al ,b, 2009Lyons et al , 2010Lyons et al und 2011. Bei den Spulwürmern ist die Resistenz gegen die makrozyklischen Laktone dagegen sehr ausgeprägt und weit verbreitet (Boersema et al 2002, Craig et al 2007, Hearn et al 2003, Kaplan et al 2006, Lindgren et al 2008, Lyons et al 2008aund b, Schougaard et al 2007, Slocombe et al 2007, Stoneham et al 2006, Veronesi et al 2009, Samson-Himmelstjerna et al 2007und 2011 (Herd 1986, Hinney et al 2011, Larsen et al 2002. Um welche insuffizienten hygienischen Maßnahmen es sich dabei handelte, wurde im Weiteren nicht diskutiert (Hinney et al 2011 …”